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Diagnosticity refers to the extent to which a source of data can discriminate between a particular hypothesis and its alternatives. In social situations, individuals often observe others' behaviors and attempt to form an impression about their personality and aptitudes. As part of this process, individuals test dispositional hypotheses, namely, hypotheses regarding others' traits and abilities. Diagnostic sources of data are those that discriminate between possessing a particular trait or ability and not possessing the trait or ability.

For example, consider a situation in which a new classmate, John, does not respond to your welcome greetings and you want to know what kind of person he is. You may generate a hypothesis (e.g., “John is unfriendly”), gather information to test this hypothesis, and draw an inference based ...